472 THE ANTIQUITY OF MAN 
" supra-plenal " ; ^ the condition in the chimpanzee and 
modern man as " sub-plenal." As regards molar develop- 
ment Eoanthropus must be placed in the plenal group — 
perhaps in the supra-plenal. 
When the chewing surfaces of the molar teeth of 
Eoanthropus are examined, it is seen that five cusps are 
clearly marked on each — two outer, two inner, and a fifth 
which is situated on the hinder border, near the junction 
of that border with the outer. The molar teeth of 
Eoanthropus were thus provided with the normal com- 
plement of cusps. It is the fifth cusp which is of chief 
interest. In the dentitions I have described as supra- 
plenal the fifth cusp reaches a higher development in 
the third molar than in the first ; in plenal dentitions 
the fifth cusp is larger on the crown of the first than of 
the third molar ; in the sub-plenal form the fifth cusp 
disappears from the last molar and becomes reduced or 
disappears from the second also. Indeed, it is not un- 
common to see modern dentitions in which the fifth cusp 
has disappeared from all the lower molar teeth. In the 
Heidelberg teeth the fifth cusp is present on the crowns of 
all the molars. In Eoanthropus we do not know what the 
condition was on the third molar, but we can see that the 
fifth cusp of the second molar, although not quite so 
large as in the first, is still well developed, and we may 
infer that it was present in the third of the series. We 
have thus evidence that the molar teeth reached a plenal 
development at least — a degree which I have not observed 
in any modern human dentition. 
A plenal development of the molar teeth must be 
regarded as a primitive feature. So, too, is the relative 
narrowness of the Piltdown molars. The length or 
proximo-distal diameter of the crowns is greater than their 
width — the measurement made between the cheek and 
tongue margins. The width of the first molar is lo mm. ; 
in the second 10*5 mm. In the gorilla and chimpanzee 
the length of the molar crowns is greater than the width ; 
1 See Keith, " Problems relating to the Teeth of the Earlier Forms of 
Prehistoric Man," Proc. Roy. Soc. of Med.., 191 3, vol. vi. p. 103. 
